Systematic Relationships Within Gelsemium (Loganiaceae): Evidence from Isozymes and Cladistics

Abstract

We used horizontal starch-gel electrophoresis to assess the degree of genetic diver- gence between four populations each of Gelsemium sempervirens and G. rankinii from the southeastern United States and to test the hypothesis that G. sempervirens is an autotetraploid derivative of G. rankinii. Levels of genetic variation in both taxa are somewhat higher than the mean for long-lived, woody perennials. There was no indication that the alleles of one species represent a subset of the variation present in the other nor was there any indication of tetrasomic expression in G. sempervirens. Moreover, chromosome counts from root tip squashes confirm that both species are diploid, with 2n = 16. There is, however, unusual mitotic behavior that may explain apparently erroneous counts of lower numbers by some previous workers, who also reported larger and fewer stomates in the presumably autotetraploid G. sempervirens. Sharp genetic differences between the two species (mean genetic identity, I = 0.48) parallel their morphological discontinuities in many characters, and there was no evidence of hybridization or introgression in regions of sympatry. A cladistic analysis using eight morphological characters supports the view that G. sempervirens and C. rankinii are sister species, more closely related to each other than either is to the third species in the genus, G. elegans from southeast Asia. Genetic evidence suggests that the lineage leading to G. rankinii and G. sempervirens split in the late Tertiary, about 3-3.5 million years ago. It is possible that G. rankinii adjusted to life in Coastal Plain swamps by loss of winged seeds, becoming adapted for dispersal by water, rather than wind.